Sampled grating distributed Bragg reflector laser controller

ABSTRACT

A controller for use with sampled grating distributed Bragg reflector (SGDBR) lasers is presented. An exemplary controller includes a table of settings representing a control surface, each setting corresponding to a separate operating point of the SGDBR laser, a first mirror current controller and a second mirror current controller. The first mirror controller and the second mirror current controller respectively control a first mirror current and a second mirror current about an estimated extremum point of the control surface to substantially maintain alignment between each of a first mirror and a second mirror, and an associated cavity mode. The first mirror current and the second mirror current can be locked at a substantially fixed distance from the extremum of the control surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of thefollowing commonly-assigned U.S. patent application which areincorporated by reference herein:

Provisional Application Ser. No. 60/291,375, filed May 15, 2001, nowabandoned and by Larry A. Coldren and Paul F. Crowder, entitled “SGDBRLASER CONTROLLER,” and

Provisional Application Ser. No.60/291,481, filed May 15, 2001, nowabandoned and by Larry A. Coldren and Michael C. Larson, entitled“CONTROLLER CALIBRATION FOR SMALL FORM FACTOR SGDBR LASER.”

This application is a continuation-in-part patent application of thefollowing co-pending and commonly-assigned U.S. patent applicationswhich are all incorporated by reference herein:

Utility Application Ser. No. 09/895,848, filed Jun. 29, 2001, by Paul F.Crowder, entitled “OPEN LOOP CONTROL OP SGDBR LASERS,” now U.S. Pat. No.6,788,719,issued Sep. 7, 2004, which claims the benefit under 35 U.S.C.§119(e) of Provisional Application Ser. No. 60/215,739, filed Jun. 29,2000, by Paul F. Crowder, entitled “OPEN LOOP CONTROL OF SGDBX LASERS.”

Utility Application Ser. No. 09/895,598, filed Jun. 29, 2001, by Paul F.Crowder, entitled “POWER AND WAVELENGTH CONTROL OF SAMPLED GRATINGDISTRIBUTED BRAGG REFLECTOR LASERS,” now U.S. Pat. No. 6,690,693, issuedFeb. 10, 2004, which claims the benefit under 35 U.S.C. §119(e) ofProvisional Application Ser. No. 60/215,739, filed Jun. 29, 2000, byPaul F. Crowder, entitled “POWER AND WAVELENGTH CONTROL OF SGDBRLASERS,” and

Utility Application Ser. No. 09/895,303, filed on Jun. 29, 2001, byGregory A. Fish and Larry A. Coldren, entitled “GAIN VOLTAGE CONTROL OFSAMPLED GRATING DISTRIBUTED BRAGG REFLECTOR LASERS,” now abandoned,which claims the benefit under 35 U.S.C. §119(e) of ProvisionalApplication Ser. No. 60/215,742, filed Jun. 29, 2000, by Paul F. Crowderand Larry A. Coldren, entitled “GAIN VOLTAGE CONTROL OF SGDBR LASERS.”

BACKGROUND OF THE INVENTION

1. Field of the Invention.

The present invention relates generally to widely-tunable semiconductorlasers. More particularly, the present invention relates toedge-emitting diode lasers. And even more particularly tosampled-grating distributed Bragg reflector (SGDBR) lasers and thecontrol thereof. Most particularly, the present invention relates to thecontrol modes of a SGDBR laser controller. The controller controls theSGDBR laser currents and temperature to therefore control laser outputpower and wavelength.

2. Description of the Related Art

Diode lasers are being used in such applications as opticalcommunications, sensors and computer systems. In such applications, itis very useful to employ lasers that can be easily adjusted to outputfrequencies across a wide wavelength range. A diode laser which can beoperated at selectably variable frequencies covering a wide wavelengthrange, i.e. a widely tunable laser, is an invaluable tool. The number ofseparate channels that can utilize a given wavelength range isexceedingly limited without such a laser. Accordingly, the number ofindividual communications paths that can exist simultaneously in asystem employing such range-limited lasers is similarly very limited.Thus, while diode lasers have provided solutions to many problems incommunications, sensors and computer system designs, they have notfulfilled their potential based on the available bandwidth afforded bylight-based systems. It is important that the number of channels beincreased in order for optical systems to be realized for many futureapplications.

For a variety of applications, it is necessary to have tunablesingle-frequency diode lasers which can select any of a wide range ofwavelengths. Such applications include sources and local oscillators incoherent lightwave communications systems, sources for othermulti-channel lightwave communication systems, and sources for use infrequency modulated sensor systems. Continuous tunability is usuallyneeded over some range of wavelengths. Continuous tuning is importantfor wavelength locking or stabilization with respect to some otherreference, and it is desirable in certain frequency shift keyingmodulation schemes.

In addition, widely tunable semiconductor lasers, such as thesampled-grating distributed-Bragg-reflector (SGDBR) laser, thegrating-coupled sampled-reflector (GCSR) laser, and vertical-cavitylasers with micro-mechanical moveable mirrors (VCSEL-MEMs) generallymust compromise their output power in order to achieve a large tuningrange. The basic function and structure of SGDBR lasers is detailed inU.S. Pat. No. 4,896,325, issued Jan. 23, 1990, to Larry A. Coldren, andentitled “MULTI-SECTION TUNABLE LASER WITH DIFFERING MULTI-ELEMENTMIRRORS”, which patent is incorporated by reference herein. Designs thatcan provide over 40 nm of tuning range have not been able to providemuch more than a couple of milliwatts of power out at the extrema oftheir tuning spectrum. However, current and future optical fibercommunication systems as well as spectroscopic applications requireoutput powers in excess of 10 mW over the full tuning band. CurrentInternational Telecommunication Union (ITU) bands are about 40 nm widenear 1.55 μm, and it is desired to have a single component that cancover at least this optical bandwidth. Systems that are to operate athigher bit rates will require more than 20 mW over the full ITU bands.Such powers are available from distributed feedback DFB) lasers, butthese can only be tuned by a couple of nanometers by adjusting theirtemperature. Thus, it is very desirable to have a source with both widetuning range (>40 nm) and high power (>20 mW) without a significantincrease in fabrication complexity over existing widely tunable designs.Furthermore, in addition to control of the output wavelength, control ofthe optical power output for a tunable laser is an equally importantendeavor as optical power determines the potential range for the laser.

Fundamentally, maximizing the output power, while stabilizing the outputwavelength and the maximizing the side mode suppression ratio are verydesirable objectives in the control of SGDBR lasers. Thus, there is aneed in the art for controllers which maximize the power output andstabilize the wavelength, particularly as the laser ages. The presentinvention meets the foregoing objectives through a novel controller.

SUMMARY OF THE INVENTION

The present invention involves the calibration of a laser and an openloop controller of the frequency (or alternatively “wavelength”) outputand power output of such a laser, which is preferably a Sampled GratingDistributed Bragg Reflector (SGDBR) semiconductor laser. The SGDBR lasergenerally includes at least four discrete sections: a gain section, aphase section, a first (or alternatively “front”) mirror section and asecond (or alternatively “rear”) mirror section. Additionally, asemiconductor optical amplifier (“SOA”) section may be included as wellas other discrete sections. The controller of such SGDBR devicesprovides stable SGDBR laser optical power and wavelength output byvarying the control currents that are applied to each of theaforementioned sections.

Calibration of the SGDBR laser and the controller establishes a table ofcurrent or voltage settings to control the laser's optical output powerand the output wavelength. Once the optical power and output wavelengthare selected, the controller of the present invention selects a set ofoperating currents or voltages from the table corresponding to theselected output power and output wavelength. Further, the controllerregulates the temperature of the SGDBR laser to a fixed, pre-selectedvalue.

A SGDBR laser generally includes a laser diode, a laser diode module(“LDM”), and a control hardware package; all of which are housed in whatshall be referred to herein as a tunable laser assembly (“TLA”). Thelaser diode is housed within the laser diode module, which may be abutterfly package or some other small package well known to thoseskilled in the art for having mounted thereto and therewithin a laserdiode.

The LDM is housed within and is a subcomponent of the TLA. Additionally,the TLA houses the controller, which comprises hardware and firmwareloaded thereupon. The TLA also comprises connectors extending betweenthe controller and the LDM providing for communication therebetween.

By properly choosing the operating currents, the current sources thatdeliver the currents to the SGDBR laser diode, and properly regulatingthe temperature of the SGDBR laser, the controller of the presentinvention provides great stability of the optical output wavelength andpower over the operating lifetime, as well as providing greaterstability over a wider range of ambient environmental conditions.

An exemplary controller embodiment of the invention includes a table ofsettings representing a control surface, each setting corresponding to aseparate operating point of the SGDBR laser, a first mirror currentcontroller and a second mirror current controller. The first mirrorcontroller and the second mirror current controller respectively controla first mirror current and a second mirror current about an estimatedextremum point of the control surface to substantially maintainalignment between each of a first mirror and a second mirror, and anassociated cavity mode. The first mirror current and the second mirrorcurrent can be locked at a substantially fixed distance from theextremum of the control surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a laser system block diagram;

FIG. 2 depicts a block diagram of an open loop control in accordancewith the present invention;

FIG. 3 is a block diagram of the current sources in accordance with thepresent invention;

FIG. 4 is a block circuit diagram of a modified Howland current sourcecircuit in accordance with the present invention;

FIG. 5 is a block diagram of a current mirror circuit in accordance withthe present invention;

FIG. 6 is a block diagram of a power and wavelength control method inaccordance with the present invention;

FIG. 7 is a block diagram of a DSP gain voltage control method for usein accordance with the present invention;

FIG. 8 is a block diagram of an analog gain voltage control method inaccordance with the present invention;

FIG. 9 is an analog phase lock circuit block diagram in accordance withthe present invention; and

FIG. 10 is a block diagram of a gain and phase current control method inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, reference is made to the accompanyingdrawings which form a part hereof, and which is shown, by way ofillustration, an embodiment of the present invention. It is understoodthat other embodiments may be utilized and structural changes may bemade without departing from the scope of the present invention.

1.0 Overview

FIG. 1 is a laser system block diagram. The present invention isdirected to the calibration of an SGDBR laser controller 100(hereinafter referred to as a “controller”). The controller 100 monitorsa multi-section, widely tunable SGDBR laser's 102 (hereinafter referredto as a “laser”) gain section voltage 104, temperature 106, andwavelength locker 108 signals. The wavelength locker signal 108 isproduced from an external reference 110 (a wavelength locker,alternatively referred to as an “FP etalon”). The laser 102 generallyhas a first or front mirror section (sometimes referred to herein as“FM”), a second or back mirror section (sometimes referred to herein as“BM”), a gain section for light generation (sometimes referred to hereinas “Gn”), and a phase section provided to tune the output wavelength ofthe laser (sometimes referred to herein as “Ph”) each controlled withcurrent inputs 112. Additionally, other sections may be incorporatedonto the laser diode including, but not limited to a semiconductoroptical amplifier, a modulator, or some other well-known component thatmay be fabricated on the same substrate as the laser.

As shown in FIG. 1, the controller 100 adjusts each section's current(with inputs 102) and the laser's temperature to maintain a fixedoptical output 114 power and wavelength. The laser's temperature isadjusted with a thermoelectric cooler 116 (or “TEC”), or some other wellknown cooling mechanism or method. The laser 102 is controlled togenerate optical output 114 at a substantially continuous power-level.

The controller 100 interfaces to a host (not shown) over a systeminterface 118, which is typically a serial or parallel interface. Thehost commands the operation of the controller 100 and may be a personalcomputer, workstation, or some other well-known device capable ofsending commands to the controller 100 through the system interface 118.

The controller 100 regulates the laser's optical output 114 power andwavelength. The controller 100 operates in one of the following controlmodes, each of which shall be described in more detail hereinbelow:

-   -   A. Open loop control using fixed operating points,    -   B. Power and wavelength control using open loop control's fixed        operating points as the initial operating points and regulating        the optical power and wavelength to a reference,    -   C. Gain voltage control using open loop control's fixed        operating points as the initial operating points and regulating        the laser mirror alignment with the cavity mode, and    -   D. Power, wavelength, and gain voltage control using open loop        control's fixed operating points as the initial operating        points.        1.1 Open Loop Control

As shown in FIG. 2, in an open loop control mode, the controller 100sets the laser optical output 114 power and wavelength by setting thelaser section (BM, Ph, Gn, FM and SOA) currents 112 from values in alook up table. It regulates the laser's temperature to a fixed value bysending control code to the TEC 116. The look-up table values aregenerated by a calibration routine. The values are fixed over thelifetime of the laser 100. The choice of the operating currents 112, thecurrent sources, and the temperature regulator guarantees maximumstability of the optical output 114 wavelength and power over the laseroperating lifetime and ambient environmental conditions.

In some embodiments of the invention, the controller can be implementedwith “open loop” controller hardware as described above, howeverfeedback is provided (e.g. to control the mirror alignment). Thus, thecontroller operates in a closed loop with respect one or more of thelaser control parameters (e.g., mirrors, gain, or phase). Control loopsfor power and/or wavelength control can also be applied. In addition,temperature regulation also can be operated under a closed loop control.As such, there is often no clear distinction between open and closedloop operation of the controller.

1.2 Operating Points

The laser operating points are typically determined by one of threecalibration routines, incremental, mirror reflectivity peak, or atwo-dimensional mirror scan.

2.0 Incremental Calibration

Incremental calibration steps and locks the laser to each InternationalTelecommunications Union (ITU) wavelength channel using a calibratedwavelength locker as a reference. It steps to the next channel byadjusting the phase current and locking the mirrors to the cavity modewith gain voltage control, which shall be discussed in further detailhereinbelow. Once at the channel, the laser wavelength is locked to thechannel by adjusting the phase current using wavelength control and thelaser power to a predetermined set point by adjusting the gain currentwith power control.

The process of incremental calibration starts with the first and secondmirrors aligned at mirror reflectivity peak 0 and then steps to locatethe next lower channel. At each cavity mode, the phase current is resetto its initial value and the search is continued. At the end of eachmirror tuning range, the mirror currents are reset to the next mirrorreflectivity peak. Once the wavelength wraps around, the process isrepeated at mirror reflectivity peak 0 by searching for the next upperchannel. The process is as follows:

For each wavelength direction about mirror reflectivity peak 0

Until (wavelength wraps),

-   -   Set gain current at nominal operational current;    -   Set mirrors at next reflectivity peak;    -   Until (end of mirror tuning range),        -   Set phase current at minimum operational current; and        -   Lock mirrors to cavity mode;        -   Until (passes cavity mode),            -   Lock power and wavelength at channel and align mirrors;            -   Record channel and currents; and            -   Step to next channel with mirrors locked to phase.                3.0 Mirror Reflectivity Peak Calibration

Mirror reflectivity peak calibration determines the mirror reflectivitypeaks, generates the mirror tuning efficiency curves, and uses thecurves to set the mirror currents for each channel. The process is asfollows:

Until (wavelength crosses mirror reflectivity peak 0),

-   -   Sweep mirror with cavity mode aligned to mirror;    -   Locate the gain voltage minima, which is the corresponding        mirror reflectivity peak; and    -   Record the currents;

Generate mirror tuning efficiency curve from reflectivity peaks;

Until (step through all channels),

-   -   Set mirrors to channel using mirror tuning efficiency curve;    -   Align phase section to the mirrors;    -   Lock wavelength to channel using wavelength control;    -   Lock power to set point using power control;    -   Record the channel and current.        4.0 Two-dimensional Mirror Scan Calibration

A two-dimensional mirror scan calibration of the present invention (asmay be employed for a small form factor TLA) determines the lasercurrents for operation at each ITU channel and the power and wavelengthand mirror control surfaces and operating points at each ITU channel Thecalibration procedure for the small form factor TLA and laser involvesthe following steps:

-   -   A. Conduct two-dimensional mirror current scan with power        leveling and wavelength locking    -   B. Channel operating region detection    -   C. Fixup of operating current values    -   D. Two-dimensional control surface characterization; and    -   E. Generate lookup table        5.0 Current Sources

As shown in FIG. 3, the controller 100 includes current sources 300which drive each of the laser's phase, mirror, amplifier, and gainsections. The current sources 300 are comprised of a voltage reference302, individual 16-bit digital-to-analog converters 304 (DACs), andvoltage-to-current (VI) converter 306. The DACs 304 connect to a digitalsignal processor (DSP) synchronous serial port (SSP) 308 through aprogrammable-logic device 310 (PLD). The PLD 310 provides a logicinterface between the DSP SSP 308 and the DACs 300. Each VI converter306 translates the corresponding DAC 304 voltage output to aproportional current that drives a corresponding laser section.

5.1 Voltage to Current Converter

As depicted in FIG. 4, a modified Howland current source (MHCS) can beused as the voltage-to-current converter 306. A current mirror 400, suchas that shown in FIG. 5, is preferably added to the output stage of theamplifier 402 to increase the drive current beyond that of the amplifier402 alone. A filter stage 404 was added at the load 406 to reduce noise.The current mirror 400 inverts the output of the amplifier 402, whichrequires the source, V_(in), at the inverting node of the amplifier 402.

The current mirror 400 operates at a fixed gain that is determined,primarily, by the ratio of the resistors 500 in the emitter leads of thetransistor 502. A resistor-capacitor (RC) compensation network 504 isadded to insure stability of the amplifier 402 and current mirror 400.The gain of the current is variable up to a maximum ratio. The maximumratio is determined by the additional drift introduced by heating of thetransistor 502 and sense resistor 506 and the maximum thermal loss thatcan be sustained by the transistor 502 and sense resistor 506. Ifadditional gain is required, an additional Q_(mo) & R_(mo) section canbe added to the mirror 400.

6.0 Power and Wavelength Control

As shown in FIG. 6, the power and wavelength controller 100 uses openloop control and feedback 600 from an external wavelength locker 602 (FPetalon) reference to lock the laser optical output power and wavelengthto the reference. Power and wavelength control compensates for drift inthe controller current sources 300 and the laser 102 operating pointsover time and temperature. The power and wavelength controls may operateindependently or interdependently.

6.1 Independent Power and Wavelength Control

The least complex control algorithm is where the controls operateindependently. Each control algorithm induces changes in one current ortemperature independent of the other. The control algorithms areclassical proportional, integral control routines. For example, thefollowing algorithm can be applied:

-   -   Optical power is adjusted by        -   Gain current (I_(gn)), or        -   Current to a SOA (if integrated into the laser).    -   Optical wavelength is adjusted by        -   Phase current (I_(ph)), or        -   Submount temperature    -   Mirror currents are left fixed.

In most cases, gain current is used on four-section devices, andamplifier current is used on five-section devices. Current to thesemiconductor optical amplifier (SOA) instead of current to the gainsection can be used in all cases concerning power control or powerleveling when an amplifier section is present on the laser chip. Gainvoltage control (See section 7) may be used in either case. However,when gain voltage control is combined with gain current-based powercontrol, power control must be interrupted (i.e. gain current heldconstant) during acquisition of a gain voltage control surface.

6.2 Interdependent Power and Wavelength Control

The independent control algorithm is slower and in its response tochanges in the optical power output and optical wavelength. The mirrorsand cavity mode become misaligned as the control algorithm adjusts thegain and phase currents from their predefined values. The quality of theoptical output may be reduced as a result of decreased side modesuppression ratio. Additionally, the probability of a mode hop(wavelength shift) is increased as the mirrors and cavity mode becomemisaligned.

The interdependent control algorithm induces primary changes in onecurrent or temperature and corrects for secondary changes in the othercurrents with an adaptive filter or estimator. This compensates forwavelength shifts or power changes and mirror misalignment induced whenthe control adjusts its primary variable. Using an interdependent powerand wavelength control algorithm as follows:

-   -   Power is adjusted by the gain current (I_(gn)),        -   Wavelength is stabilized by adjusting the phase current            (I_(ph)) by an adaptive filter; and        -   Mirror currents are realigned by a fixed estimator,    -   Wavelength is adjusted by the phase current (I_(ph)) or the        carrier temperature        -   Power is stabilized by adjusting the gain current (I_(gn))            by an adaptive filter; and        -   Mirror currents are realigned by a fixed estimator.            The interdependent controls provide more robust, stable, and            faster convergence of the power and wavelength to its            reference value.            7.0 Gain Voltage Control

Gain voltage control uses feedback from the laser gain section voltageto keep the mirrors aligned with the cavity mode. It aligns the mirrorsby minimizing the laser gain section voltage. The laser gain sectionvoltage minimum is where the cavity loss is a minimum, roughlycorresponding to maximum optical power output, wavelength stability, andside mode suppression ratio. More specifically, the gain voltage minimumcorresponds to the minimum loss condition when parasitic electricaleffects are accounted for, but gain spectrum effects offset the minimumfrom mode center in a characteristic fashion. Additional output powermay be achieved using certain techniques, such as by misaligning thefront mirror, however, in such a case, other characteristics may suffer,such as the side mode suppression ratio.

Gain voltage control can be implemented in the DSP using a numericalminima search or a least mean squares (LMS) quadratic estimator.Alternately, gain voltage control can be implemented in analog circuitryusing a phase locker circuit (PL).

7.1 DSP Gain Voltage Control

A digital signal processor (alternatively referred to as a “DSP”) may beused to implement the gain voltage control, as shown in FIG. 7. The DSPdithers the laser mirror currents 700, 702 and monitors the laser gainsection voltage 704. It uses a numerical algorithm to align the mirrorsby locating the minima of the laser gain section voltage.

7.2 DSP Minima Search Algorithm

An example minima search algorithm can be implemented as follows:

-   -   Use three data points (mirror current, gain voltage) and        estimate the slope of the gain voltage curve with respect to the        mirror current,    -   Step toward the gain voltage minima and calculate the next data        point,    -   Use the new data point and the two best points to re-estimate        the slope of the gain voltage curve,    -   Continue the above step process, continually searching for the        gain voltage minima.        7.3 DSP LMS Estimator

The minima search algorithm may be susceptible to wandering around thegain voltage minima due to noise in the sampled gain voltage signal. Thewandering is reflected as drift and noise on the optical signal. The LMSestimator reduces the wander and noise by using an array of data pointsto estimate the gain voltage surface, in effect, filtering the noise.The LMS estimator converges to the gain voltage minima faster andsmoother than the minima search.

For fixed phase and gain section currents, the gain section voltage canbe modeled using a causal Volterra series expansion over 2 inputsignals, the front mirror and back mirror currents. For ditheringsignals in the sub-100 kHz regime, the analog circuitry and the deviceitself allow a memoryless model, so a 5-tap adaptive quadratic filtermodel will suffice.

The LMS estimator can then be achieved using either of two classicadaptive filter update algorithms: a standard gradient descentadaptation (LMS or block LMS algorithm) or a recursive least squaresadaptation (RLS algorithm—based on Newton's Method).

The RLS algorithm approach is used to achieve faster convergence ofadaptive linear filters when the signals driving the system do not havesufficient spectral flatness to allow a rapid gradient descent However,in the case of adaptive linear filters, the gradient descent approachconverges just as fast as the RLS approach when white noise can be usedto drive the system. Recently published results indicate that comparablerates of convergence can be achieved with adaptive quadratic filters ifa minor filter structure modification is used and (pseudo) Gaussianwhite noise can be used to drive the system.

There are two advantages of this LMS estimator approach. First, aninitial tapvector can be stored along with the four drive currents inthe laser calibration table in flash memory (resulting in much fasterconvergence). Second, the adaptation step size can be reduced as thesystem converges, reducing steady-state misadjustment in the mirrorsection currents.

Because of the aforementioned gain spectrum effects, the optimumsetpoints for the mirror currents are actually offset from the gainvoltage minimum. Therefore, the objective is not to converge to theminimum, but to use an LMS estimator to sense where the minimum would bebased on the measured gain voltage surface in the vicinity of theoperating point. The control system adjusts the mirror currents tooperate at a calibrated current offset from the estimate of the minimum.

7.4 Exemplary LMS Estimator

An exemplary LMS estimator can use five independent data points todetermine the surface. The LMS algorithm:

-   -   Dithers the mirror currents in a linearly independent fashion        about the operating point where,        -   a point lies in each quadrant; and        -   the step size is less than the power and wavelength            accuracy;    -   Collects the gain and phase current at the mirror current when        the power and wavelength are within control tolerance;    -   Runs the LMS estimator over the data set (at least five        independent points);    -   Resets the mirror operating point to the distance from the        inflection points on the surface.

The LMS algorithm continually operates in the background and thefive-parameter fit to the quadratic control surface is:$\left. {{r \cdot \left( {f + \frac{s}{2r}} \right)^{2}} + {n \cdot \left( {b + \frac{m}{2n}} \right)^{2}} + c - \frac{s^{2}}{4 \cdot r} - {\frac{m^{2}}{4 \cdot n}\quad{simplify}}}\rightarrow{{r \cdot f^{2}} + {s \cdot f} + {n \cdot b^{2}} + {m \cdot b} + c} \right.$The parameters r and n define the surface curvature for the front andback mirror currents respectively. The parameters s and m define theoffset of the surface extremum. The parameter c defines the offset ofthe surface. The independent variables f and b are the front mirrorcurrent and the back mirror current The result maps the quadraticsurface of the gain current or phase current. The extremums are at:$\,\begin{matrix}{f = {- \frac{s}{2r}}} & {b = {- \frac{m}{2n}}}\end{matrix}$The LMS estimator that generates the surface parameters is:$\begin{pmatrix}r \\s \\n \\m \\c\end{pmatrix} = {\begin{pmatrix}{Sffff} & {Sfff} & {Sffbb} & {Sffb} & {Sff} \\{Sfff} & {Sff} & {Sfbb} & {Sfb} & {Sf} \\{Sffbb} & {Sfbb} & {Sbbbb} & {Sbbb} & {Sbb} \\{Sffb} & {Sfb} & {Sbbb} & {Sbb} & {Sb} \\{Sff} & {Sf} & {Sbb} & {Sb} & N\end{pmatrix}^{- 1} \cdot \begin{pmatrix}{Szff} \\{Szf} \\{Szbb} \\{Szb} \\{Sz}\end{pmatrix}}$where S denotes a summation over the data points of the terms multipliedtogether and z is the current of the surface. The distance is the df anddb from the extremums.

The above technique is preferably used with the gain voltage surface.Furthermore, since in the wavelength-locked case there is a significantcross term (f * b) in the gain voltage surface, a much simpler fit canbe performed independently on the front and back mirror dither usingthree fitting parameters, and the resulting extremum is calculated.

7.5 Analog Gain Voltage Control

The digital algorithms implemented in the DSP are limited in speed andaccuracy by the analog to digital converter (ADC) and digital to analogconverter (DAC) as well as the signal to noise ratio (SNR) of thecircuit.

An analog gain voltage control is set out in FIG. 8. The analog phaselocker's speed and accuracy is limited substantially only by the SNR ofthe circuit. The analog phase locker (PL) is a high speed,analog-locking loop. It can be realized by a phase lock loop (PPL) or RFdither locker. The PL works with the open loop control circuit. Theoutput of the PL adds to the output of the open loop control currentsources. For example, the gain voltage 800 can be applied to separate PLcircuits 802A, 802B of the controller 100.

As shown in FIG. 9, an exemplary PL 802 uses a high frequency narrowbandstimulus 900 to dither the mirror current The PL 802 measures the gainvoltage (V_(g)) 902 with a tuned, narrowband amplifier 904 and extractsthe phase difference between stimulus and measured signal with a phasecomparator 906. The PL 802 also drives an error amplifier that adjuststhe mirror current to the gain voltage minima and is sampled by an ADC908.

The PL error amplifier output is measured by the DSP. The DSP adjuststhe mirror current values in the open loop control lookup table toreduce the error to zero. The DSP effectively operates as an integratorfunction.

FIG. 13 illustrates the combined operation of analog gain voltagecontrol circuits to correct the outputs to the two mirrors from the openloop digital controller. The digital memory/DSP 1000 can set a firstapproximation current and voltage from a lookup table. The analogcorrection circuits 802A, 802B can provide feedback and correctionsignals to the device as described previously, and the digitalcontroller then monitors the correction signals 1002, 1004 and readjuststhe currents and voltages to have the feedback currents from the analogcorrection portions approach zero. This allows for correction of thelaser controller over the life of the SGDBR laser, and accounts forchanges in operating temperatures and conditions as well as changes inthe operation of the SGDBR laser internal components.

Gain and phase current control, such as that shown in FIG. 10, uses theextremum point (the maximum or minimum value of a function) of the gainvoltage surface (as proxy for the gain and phase current surfaces) tokeep the mirrors aligned with the cavity mode. It aligns the mirrors byoperating the mirror currents at a substantially fixed distance from thecontrol surface extremums. The distance and extremums are determinedduring calibration. The mirror operating point corresponds to best-costfunction that maximizes the optical power output, wavelength and powerstability, and side mode suppression ratio. Gain and phase currentcontrol operates in conjunction with power and wavelength control.

Gain and phase current control can be implemented in the DSP using aleast mean squares (LMS) quadratic surface estimator, such as thatpreviously described. The DSP dithers the laser mirror currents whileoperating under power and wavelength control and records the gain andphase currents when the control loops are within tolerance. It canestimate a fit to the gain voltage surface as a function of the frontand back mirror currents. Alternately, it can estimate a five-parameterfit to the quadratic control surface for the gain current and the phasecurrent as a function of the front and back mirror currents. It sets themirror currents at a distance from the surface extremums.

The power, wavelength, and gain voltage controller 100 operates thepower and wavelength control and gain voltage control simultaneously.

8.0 Conclusion

The foregoing description of the preferred embodiment of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is not intended that the scope of theinvention be limited by this detailed description.

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of the embodiments of theinvention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

1. A digital controller for a sampled grating distributed Braggreflector (SGDBR) laser, comprising: a memory storing a table ofsettings representing a control surface, each setting corresponding to aseparate operating point of the SGDBR laser; a first mirror currentcontroller; a second mirror current controller; a power controller; anda wavelength controller; wherein the first mirror current controller andthe second mirror current controller respectively control a first mirrorcurrent and a second mirror current about an estimated extremum point ofthe control surface to substantially maintain alignment between each ofa first mirror and a second mirror, and an associated cavity mode; andwherein the power controller adjusts a current to an integratedsemiconductor optical amplifier (SOA), and the wavelength controlleradjusts a current to a phase section, with error currents derived fromsignals from an external locker.
 2. The digital controller of claim 1,wherein each operating point comprises a first mirror current value, asecond mirror current value, a phase current value, an SOA current valueand a gain current value.
 3. The digital controller of claim 1, whereinthe control surface is a gain voltage surface, a gain current surface,or a phase current surface.
 4. The digital controller of claim 1,wherein the extremum is estimated using a least mean squares estimator.5. The digital controller of claim 1, wherein the first mirror currentand the second mirror current are locked at a substantially fixeddistance from the extremum of the control surface.
 6. The digitalcontroller of claim 1, wherein the first mirror current and the secondmirror current are dithered while operating under the power controllerand the wavelength controller, and wherein a gain current and a phasecurrent are recorded when the power and wavelength controllers arewithin a tolerance.
 7. The digital controller of claim 1, wherein thefirst mirror current and the second mirror current are dithered whileoperating under the power controller and the wavelength controller, andwherein a gain voltage is recorded when the power and wavelengthcontrollers are within a tolerance.
 8. The digital controller of claim1, wherein a first fit and a second fit to the control surface areestimated as a function of the first mirror current and the secondmirror current, respectively.
 9. The digital controller of claim 1,wherein the control surface comprises a quadratic control surface and afive-parameter fit to the control surface is estimated as a function ofthe first mirror current and the second mirror current.
 10. A method ofcontrolling a sampled grating distributed Bragg reflector (SGDBR) laserusing a digital controller, comprising: storing a table of settingsrepresenting a control surface in a memory, each setting correspondingto a separate operating point of the SGDBR laser; controlling a firstmirror current based on the control surface; controlling a second mirrorcurrent based on the control surface; adjusting a current to anintegrated semiconductor optical amplifier (SOA) using a powercontroller; adjusting a current to a phase section using a wavelengthcontroller; and generating a laser output of the SGDBR laser; whereinthe first mirror current and the second mirror current are respectivelycontrolled about an estimated extremum point of the control surface tosubstantially maintain alignment between each of a first mirror and asecond mirror, and an associated cavity mode to produce the laseroutput; and wherein the power controller adjusts the current to theintegrated semiconductor optical amplifier (SOA), and the wavelengthcontroller adjusts the current to the phase section, with error currentsderived from signals from an external locker.
 11. The method of claim10, wherein each operating point comprises a first mirror current value,a second mirror current value, a phase current value, an SOA currentvalue and a gain current value.
 12. The method of claim 10, wherein thecontrol surface is a gain voltage surface, a gain current surface, or aphase current surface.
 13. The method of claim 10, wherein the extremumis estimated using a least mean squares estimator.
 14. The method ofclaim 10, wherein the first mirror current and the second mirror currentare locked at a substantially fixed distance from the extremum of thecontrol surface.
 15. The method of claim 10, the first mirror currentand the second mirror current are dithered while operating under thepower controller and wavelength controller, and wherein a gain currentand a phase current are recorded when the power and wavelengthcontrollers are within a tolerance.
 16. The method of claim 10, whereinthe first mirror current and the second mirror current are ditheredwhile operating under the power controller and wavelength controller,and wherein a gain voltage is recorded when the power and wavelengthcontrollers are within a tolerance.
 17. The method of claim 10, furthercomprising estimating a first and a second fit to the control surface asa function of the first mirror current and the second mirror current,respectively.
 18. The method of claim 10, wherein the control surfacecomprises a quadratic control surface and further comprising estimatinga five-parameter fit to the control surface as a function of the firstmirror current and the second mirror current.